1. Field of the Invention:
The present invention relates to an electro-optic device formed on a silicon platform and utilized in the field of, for example, optical communication, and in particular, to a silicon-based electro-optic device formed on an SOI (Silicon-On-Insulator) substrate.
2. Description of the Related Arts:
Information communication networks typified by the Internet are constructed so as to spread throughout the world as social infrastructure essential for peoples' lives. Optical communication using optical fibers is a technique to support huge traffics on the Internet. Optical communication devices using silicon platforms can utilize a 1.3-μm band and a 1.55-μm band of wavelengths, which are included in the wavebands used for optical fiber communication. Furthermore, these optical communication devices can be manufactured by a CMOS (Complementary Metal-Oxide-Semiconductor) device fabricating processes. Thus, the optical communication devices are expected to implement high-density optical integrated circuits.
Increasing an information transmission rate per channel is a method for dealing with traffic on information communication networks, which increase year by year. An optical modulator is important for implementing this method; the optical modulator quickly converts electric signals from an LSI (Large-Scale Integration) circuit into optical signals. The LSI circuit processes information in an optical communication device. Thus, implementation of the optical modulator on a silicon platform has been proposed.
A typical proposed optical modulator is of a type that utilizes a carrier plasma effect to change the refractive index of a silicon material and thus to change propagation characteristics of light. For example, A. Liu et al. have proposed a silicon-based optical modulator using a pn junction and operated with reverse bias [A. Liu, et al., “High-speed optical modulation based on carrier depletion in a silicon waveguide,” OPTICS EXPRESS, vol. 15, no. 2, pp. 660-668 (2007)]. T. Pinguet et al. have proposed a silicon-based optical modulator compatible with fabricating processes of CMOS devices [T. Pinguet, el al., “A 1550 nm, 10 Gbps optical modulator with integrated driver in 130 nm CMOS,” Proc. of Group Four Photonics, ThA2, pp. 186-189 (2007)]. Both of these optical modulators can operate at high speed.
FIG. 1 shows the sectional structure of the optical modulator proposed by Liu et al. FIG. 1 shows a cross section of the optical modulator taken on a surface that is perpendicular to the direction in which light propagates. In the optical modulator, oxide layer 25 is formed on the top surface of silicon substrate 24, and p-doped silicon layer 23 that is an SOI layer is provided on oxide layer 25. In this case, p-doped silicon layer 23 is formed such that the sectional structure thereof includes a projecting portion serving as a core of a rib optical waveguide and slab portions arranged on the respective opposite sides of the projecting portion and connected to the projecting portion. To establish an electric connection between p-doped silicon layer 23 and electrode 27, p++-doped silicon layer 22 to which a p-type dopant of a high concentration is introduced is provided so as to connect to each of the slab portions.
On p-doped silicon layer 23, n-doped silicon layer 21 is formed such that p-doped silicon layer 23 and n-doped silicon layer 21 form a pn junction. A side portion of n-doped silicon layer 21 is connected to n++-doped silicon layer 20 to which an n-type dopant of a high concentration is introduced so as to be electrically connected to electrode 28. A portion of n-doped silicon layer 21 which contacts p-doped silicon layer 23 also forms a part of the rib waveguide. Such n-doped silicon layer 21 is formed by epitaxially growing a silicon layer on the SOI layer (i.e., p-doped silicon layer 23) and doping n-type impurities down to the vicinity of the interface between the epitaxially grown layer and the SOI layer.
Oxide layer 35 also functioning as a clad layer of the waveguide is provided so as to entirely cover p-doped silicon layer 23, p++-doped silicon layer 22, n-doped silicon layer 21, and n++-doped silicon layer 21.
In such an optical modulator, a reverse bias voltage is applied to between p-doped silicon layer 23 and n-doped silicon layer 21 via electrodes 27 and 28. Then, owing to the carrier plasma effect, a modulation operation is performed on light passing through p-doped silicon layer 23 and n-doped silicon layer 21, which form the rib waveguide.
If the optical modulation portion has such a structure, the shape of the optical modulation portion and the external electrode portion connected to the optical modulation portion is different from the structure of the waveguide connected to the electrodes. Thus, a connection structure needs to be formed which optically couples the optical modulation portion and the external electrode portion to the waveguide structure with a reduced loss and which enables high-speed electric responses.
Furthermore, in the structure shown in FIG. 1, light is confined in the rib waveguide. However, if there is only a short distance from p++-doped silicon layer 22 connected to the slab of the rib waveguide to the main body of the rib waveguide, optical absorption by the p++-doped silicon may disadvantageously result in a loss. Specifically, the p++-doped silicon has a higher optical absorbance than p+-doped silicon or pure silicon (or intrinsic silicon), which has a lower dopant concentration. Thus, the p++-doped silicon cannot be used in the central portion of the rib waveguide. Besides the central portion of the rib waveguide, the vicinity of the rib structure is not preferable as a position where the p++-doped silicon is placed, in terms of a reduction of optical loss.
On the other hand, to allow the optical modulator to operate at high speed, the electrodes are desirably arranged near the rib structure to reduce electric resistance associated with the electrodes. Hence, arranging the electrodes near the rib waveguide in order to increase the speed of the electro-optic device is in a tradeoff relationship with arranging the electrodes away from the rib waveguide in order to reduce an optical loss in the electro-optical device.
In the configuration in which electrode 27 is connected to p++-doped silicon layer 22 as shown in FIG. 1, the following problem may occur if the thickness of p++-doped silicon layer 22 is reduced in order to enhance confinement of light in the rib waveguide structure to increase efficiency. That is, a manufacturing variation is likely to occur in the thickness of a silicide layer serving as a contact layer between electrode 27 and p++-doped silicon layer 22. This makes a contact resistance unstable.
In the optical modulator shown in FIG. 1, a pn junction is formed. However, WO2004/088394 discloses an example of a silicon-based electro-optic modulator in which a SIS (Semiconductor-Insulator-Semiconductor) junction is formed instead of the pn junction. This optical modulator has a waveguide structure in which a p-doped silicon layer and an n-doped silicon layer are stacked via a relatively thin dielectric layer. When a modulation signal is applied to between the two silicon layers, free carriers are accumulated or removed or the carrier concentration is reversed, on the respective opposite sides of the dielectric layer. This changes an effective optical refractive index.
FIG. 2 shows the sectional structure of the optical modulator proposed by Pinguet et al. FIG. 2 shows a cross section of the optical modulator taken across a surface perpendicular to the direction in which light propagates. This optical modulator also includes a rib waveguide of an SOI structure but is different from the one shown in FIG. 1 in that a p-doped silicon layer and an n-doped silicon layer are arranged so as to form a lateral pn junction along a centerline extending in the longitudinal direction of the rib waveguide such that the plane of the pn junction is perpendicular to the silicon substrate. Each of the doped silicon layers has a low impurity concentration in the rib waveguide portion. In the slab portion of the rib waveguide, the impurity concentration increases with the distance from the rib waveguide. Pinguet et al. fail to fully describe the specific structure of the optical modulator. However, p+-doped silicon layer 32 and n+-doped silicon layer 30 have larger thicknesses at each position in the slab portion where the slab portion contacts the electrode. In this structure, a stable silicide layer is expected to be formed so as to serve as a contact layer.
In the optical modulator or electro-optic device including the rib waveguide of the SOI structure, it is completely unclear how close the contact layer is to be located with respect to the rib waveguide for a high speed operation and how, in association with connection to the electrode, the contact layer is to be located with respect to the central portion of the waveguide, through which light passes. Furthermore, silicon blocks provided on the slab portions on the respective opposite sides of the rib waveguide may cause a propagation loss or an insertion loss resulting from stray light.